In general, a ceramic sheet is produced by the following steps. Ceramic powder is mixed with an organic binder, a solvent, and if necessary, a plasticizer and a dispersant, and the resultant mixture is kneaded to produce a slurry. The slurry is formed into a green sheet by a method such as doctor blade process, calendering process and extrusion process. The green sheet is stamped or cut into a predetermined shape and baked to produce a ceramic sheet.
The ceramic sheet is excellent in mechanical strength, toughness, abrasion resistance, chemical resistance, corrosion resistance, heat resistance and electric insulation. For these advantages, the ceramic sheet is used in applications including hybrid integrated circuit boards, outer plates of heat resistant and various structural members such as fire resistant boards, and sliding members. In addition, taking advantage of its oxygen ion conductivity, the ceramic sheet is also used in applications such as solid electrolytes of oxygen sensors, humidity sensors and fuel cells and the like.
In order to use the ceramic sheet in the above-described applications, the sheet is required to have high uniformity over its entire surface with few defects such as foreign matters and flaws. The defects such as foreign matters and flaws is likely to lower the mechanical strength of the ceramic sheet and cause variations in the strength over the surface thereof. In this case, the ceramic sheet does not exhibit excellent and uniform properties over its surface. In particular, the presence of the foreign matters and flaws is a serious problem, when the ceramic sheet is used as a solid electrolyte in fuel cell in which the ceramic sheets are stacked. In the fuel cell, the ceramic sheet is kept at a high temperature of about 800 to 1000° C. under the load of at least 10 g/cm2 for a long period of time. Under this condition, foreign matters and flaws in the ceramic sheet is likely to affect the Weibull modulus of the sheet, which indicates the mechanical strength and variation in mechanical strength.
The defects such as foreign matters and flaws impair not only the mechanical strength and uniformity of properties of the ceramic sheet but also the electric characteristics thereof. For example, when the ceramic sheet is a thin film having an area of 100 cm2 or larger and a thickness of 0.3 mm or smaller, the thin ceramic sheet is likely to be cracked due to foreign matters and flaws, resulting in deteriorating electric characteristics thereof. In addition, the conductivity of the ceramic sheet is likely to decrease by the foreign matters in the ceramic sheet, or by a product produced by the solid phase reaction between the foreign matters and the zirconia (a main component of the ceramic sheet) due to exposure of the ceramic sheet to high temperature for a long period of time. Furthermore, the difference of thermal expansions between the zirconia and the foreign matters may cause cracks to the ceramic sheet. When the ceramic sheet is used in a fuel cell device which is sealed, the foreign matters and flaws may also cause another problem as follows. Since the periphery of the ceramic sheet is firmly fixed in the fuel cell, thermal expansion of the ceramic sheet due to exposure to high temperature may generate a stress, to cause cracks to the sheet itself. Thus, foreign matters or flaws impair the characteristics of the ceramic sheet for a solid electrolyte.
For the above reasons, the ceramic sheet is required to minimize the number of defects such as foreign matters and flaws. However, it is inevitable for the obtained ceramic sheet to have defects for the following reasons.
A green sheet contains organic components such as a solvent, a binder and a plasticizer. The organic components contain foreign matters in some cases. When the green sheet is baked, although the organic components (e.g., a solvent) themselves can decompose to be removed, the foreign matters cannot decompose, to thereby remain in the ceramic sheet as defects.
In a furnace, particles of foreign matters and dusts fly in all directions by a convective atmospheric gas. When the particles of foreign matters and dusts fall onto or adhere to the green sheet during the baking the green sheet, they remain in the resulting ceramic sheet as defects.
The contamination of foreign matters into the green sheet during baking can be suppressed to some extent by removing them in the process of forming the green sheet and cleaning the furnace. However, it is still difficult to avoid the formation of flaws for the following reasons.
At present, in order to increase the productivity of a thin ceramic sheet having an area of 100 cm2 or larger and a thickness of 0.3 mm or smaller, a method such as illustrated in FIG. 1 has been studied. A cover 3a in the form of sheet containing ceramics as a main component is placed on a ceramic setter 1. On the cover 3a, green sheets to be baked (hereinafter, referred to as “a green sheet for ceramic sheet”) 2 and spacers 3 in the form of sheet containing ceramics as a main component are alternately stacked. On the top green sheet for ceramic sheet 2, a thick cover 3b serving as both a cover and a weight is placed. In this state, the green sheets for ceramic sheet 2 are baked. This method is advantageous in that a number of green sheets 2 can be baked at the same time. The variations in qualities among the green sheets 2 can be suppressed by using porous sheets as the spacers 3 to release an thermally decomposed organic binder from the green sheets during dewaxing of the sheets.
This method, however, has the following disadvantage. In the baking, the green sheet 2 shrinks when the organic binder contained in the green sheet 2 decomposes to be removed from the green sheets 2 and when the ceramic powder contained in the green sheet 2 is sintered. In the method illustrated in FIG. 1 where the plurality of green sheets 2 are stacked and baked, each green sheet 2 shrinks while being scraped by the spacer 3 and/or the covers 3a, 3b in contact with the green sheet 2. The scraping forms strapes, stripe flaws, convex and concave flaws on the resulting ceramic sheet surface. In order to satisfy the demand for high productivity by employing the method of baking green sheets in a stacked state, the above defects are not avoidable. The number and size of the defects increase as the ceramic sheet has a larger area.
In an attempt to suppress the generation of defects in baking a plurality of stacked green sheets, Japanese Unexamined Patent Publication No. 4-160065 discloses a method which uses, as a spacer, a green sheet containing inorganic powder having an average particle diameter of 5 to 300 μm in a dispersed state and having a surface roughness of its one side of 10 to 200 μm.
However, in this prior art method, it is still difficult to produce a ceramic sheet having uniform quality over its entire surface with a decreased number of defects such as flaws and foreign matters, in particular, a thin ceramic sheet having an area of 100 cm2 or larger and a thickness of 0.3 mm or smaller.
The present invention has been conducted to solve the above-described problems, and the objective thereof is to provide a ceramic sheet having uniform quality over its entire surface with a decreased number of defects such as foreign matters and flaws, in particular a thin and large ceramic sheet suitable as a solid electrolyte of fuel cell. The another objective of the present invention is to provide a method for producing the above-described ceramic sheets by baking a plurality of green sheets while effectively preventing the generation of defects such as flaws.